This was driven by some quirky nonsense whereby

• undici's type definitions use the File interface defined by @types/node (which, like TypeScript's own web libraries, doesn't declare a toStringTag property)
• within the repo itself, the project's devDependencies use @types/node 22.x, which depends on the outdated undici-types 6.21.0 from 2024
• this version of undici-types is old enough to have had its own File implementation (which declared a toStringTag property)
• in the tsd test environment, it's possible to accidentally unearth the File interface from the outdated undici-types, if it's resolved via a global fetch type rather than an imported one (since TS resolves the globals to the undici-types dependency in node_modules, not the local project)
• it was noticed in the test environment that if one manages to observe this old undici-types File interface by deliberately targeting globalThis, then it's not assignable from the one used in the undici project itself (the @types/node one), due to the missing toStringTag property – even though this is a meaningless observation
• the solution for this non-issue was to (breakingly) externally monkeypatch @types/node from undici, in order to make its File interface assignable to the one that no longer exists

Let's not do this again.

Refs: #4867 (comment)